The invention relates to a device for monitoring a vital sign of a living subject, comprising a measuring unit for measuring at least one physical quantity indicative of said vital sign at an area overlying an artery of the subject, which measuring unit is provided on a carrier to be worn by the subject during operation, and a processing unit for processing the measured physical quantity into a value for the vital sign.
A device of the type defined in the opening paragraph is known from U.S. Pat. No. 4,660,566.
The known device is a device for monitoring a vital sign, the vital sign monitored by this device being a blood pressure of a subject. The known device comprises a measuring unit comprising a sensor provided on an inflatable cushion, and a processing unit. This cushion is provided on a carrier comprising a bracelet of flexible material worn by a user around his or her wrist. The sensor measures a physical quantity indicative of the blood pressure at an area overlying an artery of the subject. To measure the physical quantity effectively, the sensor on the cushion has to be positioned correctly relative to the artery. Therefore the sensor on the cushion is positioned relative to the artery on the wrist during operation, by means of a strap which is wound around the thumb of the subject and which is interlocked with a strip on the bracelet. Further locking strips are attached to the flexible bracelet to maintain the bracelet in this position. In this known device, pressure pulses are applied by means of the cushion to a local, discrete external point overlying the radial artery of the subject so as to effect a blocking or an unblocking condition of the artery. When the sensor detects said condition, the instantaneous value of the pressure in the cushion is measured, for providing through the processing unit a value of said subject""s blood pressure. A disadvantage of the known device is that a correct positioning of the sensor relative to the artery is not guaranteed, and thus a xe2x80x9ccorrectxe2x80x9d measurement could in fact be incorrect. This is highly undesirable both in the monitoring of a vital sign by a professional in a professional situation, and in a domestic situation, where the measurement is carried out by a person him- or herself.
It is an object of the invention to provide an improved device for monitoring a vital sign of a subject which is easy to use and is correctly placeable relative to the artery.
To achieve this object, a device according to the invention is characterized in that the device is provided with at least one detector for detecting the position of the measuring unit relative to the artery of the subject during operation and for actuating a feedback signal in dependence on this position. A user of the device knows that a correct position of the measuring unit relative to the artery is reached when the feedback signal is given. The user can move the device relative to the artery until the feedback signal is given, and thus a correct positioning of the measuring unit relative to the artery is achieved in an easy manner. Next to that, the structure of the device is simpler, because no additional fastening parts are necessary in the device for achieving a correct positioning of the measuring unit. An embodiment of a device in accordance with the invention is characterized in that the detector comprises at least one sensor for detecting the presence of the artery in the vicinity of the sensor, which sensor is chosen from a group comprising electro-optical sensors and ultrasound sensors. These sensors work efficiently and can be easily integrated within the device, while the comfort of use of the device is maintained.
It is furthermore advantageous when a plurality of sensors for detecting the presence of the artery in the vicinity of the sensor are provided in an array. One given sensor in the array will detect the presence of the artery in the case of a correct position of the measuring unit relative to the artery. A number of sensors in the array will not detect the presence of the artery, but at least one sensor will when the device is placed with its measuring unit on the artery. Thus an indication can be given to the user through the sensors in which directions the device should be moved for said given sensor to detect the presence of the artery. Furthermore, the detector can be varied with respect to its structure and size through combining of arrays of sensors, depending on the type of device for monitoring vital signs in which the detector is applied.
An embodiment of a device in accordance with the invention is characterized in that the feedback signal is chosen from a group comprising audio and visual signals. Audio and visual signals are easily recognizable for users. Furthermore, the freedom is created to adapt the feedback signal to the demands of the varying types of devices for monitoring vital signs and to the demands of varying user groups of the device.
It is advantageous when the carrier comprises an annular body which is at least partly rigid, and especially when the annular body is entirely rigid. The rigid annular body simplifies the placement of the measuring unit on the subject whose vital signs are to be monitored. The rigid annular body has a fixed shape corresponding to the body part of the subject it is to be placed on, so that its placement can be performed in a clear and unambiguous way. Next to that, the rigid annular body contributes to the correct positioning of the measuring unit relative to the artery of the subject. It offers a first rough positioning of the annular body with the measuring unit on the subject, while the positioning of the measuring unit relative to the artery of the subject is further refined by shifting of the annular body relative to the artery over small distances in reaction to the feedback signal actuated by the detector. Furthermore, particularly if the measuring unit comprises an inflatable cushion, the rigid annular body bounds the level of expansion of the inflatable cushion, so that the cushion cannot be excessively inflated during operation. This benefits an effective application of pressure by the cushion to the area overlying the artery.
An embodiment of a device in accordance with the invention is characterized in that the measuring unit comprises an inflatable cushion for applying a pressure to said area overlying said artery of the subject, and the cushion comprises a plurality of separately inflatable cushion parts. During operation each cushion part can apply pressure locally on said area overlying said artery of the subject. The transmission of said pressure is thus less disturbed by the structure of the area, comprising bones and tendons, because the cushion parts can enter between the bones and tendons. Once the cushion has been positioned correctly relative to the artery with use of the detector, this pressure is applied to the area overlying the artery of the subject in an efficient way.
An embodiment of a device in accordance with the invention is characterized in that the cushion parts comprise protuberances, projecting from a surface area of the carrier in a direction practically perpendicular to that surface area. In this way, the separately inflatable cushion parts are finger-shaped elements protruding from the carrier, which benefits an efficient application of local pressure by each cushion part to said area overlying said artery of the subject.
With the cushion comprising a plurality of separately inflatable cushion parts, it is advantageous when the carrier comprises an annular body which can be closed during operation, and when the cushion is provided along an entire inner wall of said body. The cushion parts penetrating between bones and tendons are also advantageous for applying an annular pressure to an extremity of a subject, as in known other blood pressure monitors.